1. Field of the Invention
This invention relates to a toner having at least a fine silica powder, which is favorably usable when electrical latent images are formed and developed in electrophotography, electrostatic printing or toner jet recording; and an image forming method making use of the toner.
2. Description of the Related Art
Conventionally, electrophotography is a process in which a recorded image is obtained by forming an electrostatic latent image on a latent image bearing member (photosensitive member) by various means, subsequently developing the latent image by the use of a toner to form a toner image on the photosensitive member, transferring the toner image to a recording material by the use of a direct or indirect means as occasion calls, followed by fixing by the action of heat, pressure and/or light.
As developing systems, conventionally a one-component developing system and a two-component developing system are available. In either developing system, as printers, or copying machines, for business use or personal use which are operated by electrophotography, there is in recent years an increasing demand for making them smaller in size, higher in speed, longer in lifetime (making stable images obtainable over long-term use) and so forth.
As toners for electrophotography which are commonly used in both the one-component developing system and the two-component developing system, they use a surface-treated fine powder of silica, titanium oxide or alumina, having been subjected to hydrophobic treatment, for the purposes of providing the toners with fluidity and charge stability and making them less adherent to structural members.
A common one-component developing system is a system in which recorded images are obtained by bringing a toner carrying member on the surface of which a toner is coated in the form of a thin layer and an electrostatic latent image bearing member into contact with each other to render electrostatic latent images visible and further transferring and fixing the resultant visible images one after another onto a recording material. Here, the toner assumes any desired state of charging, where in such charging the toner is provided with charge by forming a thin layer of the toner on the toner carrying member surface by the aid of a layer thickness control member and simultaneously bringing the toner into friction with the toner carrying member surface and the control member surface. Further, the charge polarity of this toner is utilized to render the electrostatic latent images visible potentially by utilizing an electric field at a developing zone.
Hence, when the thin layer of the toner is formed on the toner carrying member surface by the control member, the toner and/or an external additive such as a fine silica powder tend(s) to come to melt-stick to the toner carrying member surface and control member surface because of pressure put by the control member. As the result, any toner layer disorder due to the matter having thus melt-stuck thereto may appear on images to tend to cause line marks (development line marks) on the images. Accordingly, a toner and/or an external additive such as a fine silica powder is/are desired which can not easily melt-stick to the toner carrying member surface and control member surface.
In the two-component developing system, the toner and/or the external additive such as a fine silica powder tend(s) to come to melt-stick to carrier particles as a result of long-term service. As the result, such matter having melt-stuck thereto tends to cause a lowering of charge-providing ability of the carrier to the toner, so that in some cases the charge quantity of toner can not stabilize to make image density unstable or cause fog seriously and make it unable to obtain stable images over a long period of time. Accordingly, a toner and/or an external additive such as a fine silica powder is/are desired which can not easily melt-stick to the carrier particles.
Meanwhile, as conventional fine silica powders, fine silica powders the particle surfaces of which have been subjected to hydrophobic treatment are known in the art (see, e.g., Japanese Patent Publication No. S54-016219 and Japanese Patent Laid-open Applications No. S59-201063 and No. S55-120041). These hydrophobic fine silica powders are those having been treated with dimethyldichlorosilane or hexamethyldisilazane, and can not be said to be sufficiently hydrophobic, thus, in a severe high-temperature and high-humidity environment, these may cause a lowering of charge quantity because of moisture absorption. As the result, a problem tends to come about such that, as a result of long-term service, image density becomes unstable or fog occurs seriously.
A method is also disclosed in which a fine silica powder is treated with silicone oil and used in a toner (see, e.g., Japanese Patent Laid-open Application No. S49-42354). This method can ensure hydrophobicity to a certain degree. However, since the silicone oil is a high-molecular substance, agglomeration takes place when the fine silica powder is treated with the silicone oil, to produce agglomerates of about 200 μm in size or produce powder lumps of various sizes upon further mutual agglomeration of such agglomerates. As the result, the toner may have a poor fluidity to tend to cause fog.
Such a surface-treated fine silica powder has an average primary particle diameter of approximately from a few nm to tens of nm, whereas the fine silica powder is, as a state before mixing by its external addition to toner particles, present in the form of about 200-μm agglomerates of primary particles, or powder lumps formed upon further mutual agglomeration of such agglomerates. In particular, the fine silica powder treated with what is of a silicone oil type is so strongly agglomerative between primary particles or agglomerates as to have a tendency to easily melt-stick to the toner carrying member and control member or to the carrier particles, and so forth.
Accordingly, in order to stabilize surface treatment performance, a method is proposed in which, aiming to keep particles from agglomerating to have low fluidity or dispersibility when a treating agent is used in a large quantity, the surface-treated fine silica powder is used after it has been disintegrated (see, e.g., Japanese Patent Laid-open Applications No. H08-152742 and No. 2004-168559).
For example, in the above Japanese Patent Laid-open Application No. H08-152742, it is disclosed that a surface-treated fine powder is used after it has been disintegrated by means of a jet mill. However, such a powder subjected to disintegration treatment has a portion remaining untreated, and hence, though it is temporarily made fine, has a problem that it may again agglomerate with lapse of time. As the result, in long-term service, the fine silica powder tends to come liberated from toner, where the fine silica powder having come liberated therefrom tends to adhere or melt-stick to the toner carrying member and control member in the case of the one-component developing system, or to carrier particles in the case of the two-component developing system, tending to cause difficulties in images.
For another example, in the above Japanese Patent Laid-open Application No. 2004-168559, a fine silica powder is disclosed which has been subjected to disintegration treatment until its agglomerates come very fine, so as to have particle size distribution in a specific range. However, where the powder subjected to disintegration treatment in this way is mixed in the toner by external addition, the fine silica powder tends to come buried in toner particles during long-term service because the agglomerates have been too finely disintegrated. As the result, as a toner, it tends to become greatly low in fluidity to become poor in transfer performance, or the charge quantity of toner tends not to stabilize to make image density unstable or cause fog seriously.
Thus, it has been difficult to make the charge quantity of toner stabilize in every environment and also to keep the toner and/or the fine silica powder from melt-sticking to the toner carrying member and control member or to the carrier particles.